1. Field of the Invention
The present invention relates to a mobile radio communication device, and in particular relates to a mobile radio communication device wherein a digital communication system of the time division multiple access (TDMA) type is adopted.
2. Description of the Related Art
In a radio communication system of the TDMA (time division multiple access) type, the signals of a plurality of mobile stations are transmitted with time division on the same frequency. For example, in a four-channel multiplex TDMA system, a single frame is divided into four time slots and the signal of a single mobile station is allocated to one time slot, so that signals of a maximum of four mobile stations can be transmitted with the same frequency.
At the receiving end, it is arranged for only the signals of the transmitting party to be received by effecting synchronization with the time slot of the transmission end, so that frame-synchronized received signals can be received. In more detail, in a digital communication system of this type, the base station or network accommodating the base station determines the TDMA timing such as that of the frames and/or time slots, and the mobile station is operated in synchronism with this timing.
However, in the case of a mobile station just after it has been switched on or just after it is moved into the radio service area from outside it, the reception clock will not be synchronized with the transmission clock of the base station and the TDMA timing will not be known. The mobile station therefore first of all continuously receives the frequency in question (asynchronous reception), waiting for a signal from the base station.
The signal from the base station contains preamble bits repeated in a fixed pattern; the mobile station regenerates the transmission clock of the base station by receiving these and performs an operation to pull in the reception clock by synchronizing its own reception clock with this.
In addition, the signal from the base station contains synchronization bits; the mobile station receives these bits with the reception clock synchronized to the base station. The mobile station then performs synchronization establishment operation in which it seizes the TDMA timing of the base station from the position of the synchronization bits received and the frame structure, which are laid down beforehand; subsequently, it shifts to synchronous reception of only this time slot transmitted by the base station.
Also, on synchronization reception, it performs a synchronization maintenance operation to maintain the TDMA timing produced by demodulation of the reception clock and synchronization bit reception.
However, in the above asynchronous reception, if the mobile station spuriously recognises noise as a signal from the base station, the base station may shift to synchronized reception with the wrong TDMA timing. As a result, it will only detect failure of synchronization on failure of reception a fixed number of times as the synchronization signal which it ought not to be receiving is repeated; it will then return to asynchronous reception.
Consequently, some considerable time may elapse before synchronization with the base station is established and communication becomes possible. In particular, since noise has a random pattern, the probability of such spurious operation occurring is quite high.
In order to prevent this, in conventional radio communication systems, there was provided means that monitored the level of the RSSI (received signal strength indicator) signal that was output from the radio receiver of the mobile station during asynchronous reception, and that performed pull-in operation of the reception clock and establishment of synchronization only when the level of this signal exceeded a threshold value as a result of which it was judged that sufficient sensitivity of reception existed.
Also, there was provided means that, also during synchronous reception, constantly monitored the level of the RSSI signal at the mobile station so as to exclude the possibility of the mobile station operating spuriously in response to reception of noise rather than the signal from the base station, and which maintained synchronization only whilst this threshold value was exceeded.
That is, in the conventional radio communication system, an RSSI signal that was output from the radio receiver of the mobile station was input to an analog signal comparator and the result of comparison with a threshold value that decided whether sufficient reception sensitivity existed was output, ON/OFF on establishment of synchronization and maintenance of synchronization being controlled by this output signal.
However, when a mobile station is searching for a base station under asynchronous reception, if it finds and connects up to a base station of low reception level, subsequent synchronization maintenance in synchronized reception is difficult and there is a possibility of communication becoming impossible whilst service is in progress. In order to prevent this, it is necessary to set the threshold value of the RSSI signal level that determines reception clock pull-in during asynchronous reception and ON/OFF of synchronization establishment on the high side.
On the other hand, in order to extend the distance over which communication is possible after shifting to synchronous reception, it is necessary to maintain synchronization even though the signal level from the base station has become somewhat weaker: it is therefore necessary to keep the threshold level of the RSSI signal that determines ON/OFF of synchronization maintenance during synchronous reception as low as possible.
Thus, the requirements in respect of the RSSI signal level threshold value are exactly opposite for asynchronous reception and synchronous reception; as shown in FIG. 5, with the conventional radio communication system that employed only a single analog signal comparator one or other of these demands had to be sacrificed.
That is, conventionally, the threshold value was normally set to a low level, giving priority to maintenance of service; however, in this case, there was a high probability of connecting to radio base stations where reception conditions were not really very good.